diff --git a/contrib/usart-stm32f103.c b/contrib/usart-stm32f103.c
index 7ab9b80..053e513 100644
--- a/contrib/usart-stm32f103.c
+++ b/contrib/usart-stm32f103.c
@@ -1,7 +1,7 @@
 /*
  * usart-stm32.c - USART driver for STM32F103 (USART2 and USART3)
  *
- * Copyright (C) 2017  g10 Code GmbH
+ * Copyright (C) 2017, 2019  g10 Code GmbH
  * Author: NIIBE Yutaka <gniibe@fsij.org>
  *
  * This file is a part of Chopstx, a thread library for embedded.
diff --git a/contrib/usart.h b/contrib/usart.h
index 7c982e6..8b16acd 100644
--- a/contrib/usart.h
+++ b/contrib/usart.h
@@ -45,9 +45,9 @@ PAR_BITS  3
 
 /* non-POSIX, non-USB-CDC configs */
 #define MODE_SMARTCARD (1 << 30)
-#define MODE_IRDA      (2 << 30)
-#define MODE_IRDA_LP   (3 << 30)
-#define MASK_MODE      (0x3 << 30)
+#define MODE_IRDA      (2UL << 30)
+#define MODE_IRDA_LP   (3UL << 30)
+#define MASK_MODE      (0x3UL << 30)
 /* 0: standard, 1: smartcard, 2: IrDA, 3: IrDA-LP */
 
 struct usart_stat {